Exterior trim can easily degrade over time with exposure to sunlight and extremes of hot and cold, but it depends to what extent it has degraded. If the colour has faded then a suitable trim restorer from a car accessory shop should bring back its original shade. However if it has degraded to the point where moisture is getting into the vehicle or it is no longer watertight, then replacement is the only option. If your local garage is quoting a high price then we would suggest trying alternatives, such as a local Nissan specialist or independent automotive glass specialist, who may be able to offer a repair at a reduced price.